A conventional DC-to-DC converter, as shown in FIG. 1, has a switching transistor Q connected through the primary winding of a transformer T between the positive and negative inputs. The switching transistor Q is enabled by a gate or base drive circuit 10, which in turn is enabled by a power supply control circuit 11. DC output voltage is referenced from an output control circuit 12. An isolated coupling 13 isolates the primary and secondary sides of the circuit and carries a control signal 14a-b--which is typically an analog voltage--between output control circuit 12 and power supply control circuit 11.
Unfortunately, such isolated couplings have many drawbacks. Opto-couplers, which are frequently used, suffer from variations in the coefficient of coupling between individual units, non-linearity, poor frequency response and a susceptibility to false signals due to high frequency voltage transients between primary and secondary circuits. A variety of alternative transformer couplings typically suffer from poor frequency response, beating effects with the main switching frequency, and sometimes produce a high frequency ripple which introduces noise in the output voltage.
Therefore, it would be highly desirable to provide an improved method and apparatus for coupling the primary and secondary stages of a switching power supply.